1. Field of the Invention
The present invention relates to a developing device that develops a latent image formed on a latent-image carrying member by applying toner hopping on a surface of a toner carrying member to the latent image, a process cartridge, and an image forming apparatus.
2. Description of the Related Art
A developing device that develops a latent image formed on a latent-image carrying member by applying toner hopping on a surface of a cylindrical toner carrying member to the latent image is disclosed in Japanese Patent Application Laid-open No. 2007-133389. A plurality of long electrodes each extending in the axial direction is arranged on the toner carrying member in the circumferential direction at a predetermined pitch. An alternating electric field is formed between the adjacent electrodes on the surface of the toner carrying member. The toner moves back and forth between the adjacent electrodes by hopping in accordance with change in a direction of the alternating electric field. The toner repeatedly hops between the adjacent electrodes while the toner is conveyed to a developing area where the toner carrying member is opposed to a latent-image carrying member in accordance with the rotation of the toner carrying member. When the toner hops from the surface of the toner carrying member at the developing area and floats near the surface of the latent-image carrying member, the toner is attracted by an electric field formed by the latent image whereby the toner adheres to the latent image. In this manner, a toner image is formed on the surface of the latent-image carrying member.
In a conventional developing device, toner is conveyed to a developing area such that the toner is moved in a certain direction by hopping on a surface of a toner carrying member, instead of conveying the toner to the developing area in accordance with the surface movement of the toner carrying member while the toner is hopping between the electrodes. For example, Japanese Patent Application Laid-open No. 2004-198675 discloses a developing device that employs a toner carrying member on which three electrode, a A-phase electrode, a B-phase electrode, and a C-phase electrode are repeatedly arranged in this order. The toner is caused to hop on the surface of the toner carrying member sequentially from the A-phase electrode to the B-phase electrode, from the B-phase electrode to the C-phase electrode, and from the C-phase electrode to the A-phase electrode, so that the toner is conveyed to a developing area.
Such a developing device employing a system in which the hopping toner is used for development (hereinafter, “the hopping system”) makes it possible to develop an image with a low electric potential, which cannot be achieved in a conventional one-component developing system or a conventional two-component developing system. In the hopping system, for example, the toner can selectively adhere to an electrostatic latent image having a potential difference of only several tens of volts (V) from a non-image area formed around the electrostatic latent image.
However, an insufficient hopping height of the toner on the surface of the toner carrying member causes development failure of an isolated dot. Specifically, if the hopping height of the toner is lower at the developing area, a distance between the toner hopping on the surface of the toner carrying member and the surface of the latent-image carrying member is larger. As a result, it is difficult for the toner to adhere to the electrostatic latent image formed on the surface of the latent-image carrying member. A relatively high electric field is formed at an area where a plurality of image dots is arranged in series on the surface of the latent-image carrying member due to a plurality of latent images corresponding to the image dots. Therefore, even if the distance between the toner hopping on the surface of the toner carrying member and the surface of the latent-image carrying member is relatively large because of the relatively low hopping height of the toner at the developing area, the toner can be attracted by the electric field whereby the toner can adhere to the latent images. However, the intensity of the electric field is not so high at an area where only one image dot is present in an isolated manner on the surface of the latent-image carrying member. Therefore, if the hopping height of the toner is relatively low at the developing area, the toner is not properly attracted by the electric field, resulting in development failure of the isolated dot.
If the intensity of the electric field formed on the surface of the toner carrying member is high enough to obtain a sufficient hopping height of the toner, the isolated dot can be developed in an improved manner. However, the toner hopping high on the surface of the toner carrying member is easily splattered by falling out of the electric field due to an air current, inertia, a surrounding environment, or the like. Especially, the toner is easily splattered because the toner falls out of the electric field formed between the electrodes at ends on the surface of the toner carrying member in a direction perpendicular to a direction in which the electrodes are arranged even if the hopping direction of the toner slightly shifts from the direction in which the electrodes are arranged.
Japanese Patent Application Laid-open No. 2002-351218 discloses a developing device in which the toner is prevented from splattering from the surface of the toner carrying member. Specifically, the developing device includes, as a toner carrying member, a flat board on which a plurality of rectangular electrodes each extending in a width direction of the flat board is arranged at a predetermined pitch in a longitudinal direction of the flat board. The developing device causes the toner to sequentially move from one end to the other end of the flat board in the longitudinal direction by hopping, so that the toner is conveyed to a developing area. An electrode substrate is opposed to an area other than the developing area on the surface of the toner carrying member with a predetermined gap. The electrode substrate limits a hopping height of the toner, so that it is possible to prevent the splattering of the toner due to a high hopping height of the toner. Because ends of the electrode substrate in its width direction are curved toward the toner carrying member, a distance between the electrode substrate and the toner carrying member is smaller at the ends than at a middle area in the width direction. With this configuration, an electric field formed at the ends of the surface of the toner carrying member in the width direction is oriented in a direction from the ends toward the middle area. Thus, when the toner is about to move from above the toner carrying member outward by hopping, the toner is attracted by the electric field at the ends of the toner carrying member in the width direction and moved back to an area above the toner carrying member whereby the splattering of the toner in the width direction is prevented. With the developing device having the above configuration, even if the intensity of the electric field is relatively high enough to develop the isolated dot in a proper manner, it is possible to prevent the splattering of the toner.
However, the arrangement of the electrode substrate makes the configuration of the developing device complicated. Especially, if the cylindrical toner carrying member is employed as described in Japanese Patent Application Laid-open No. 2007-133389, the outer surface of the cylindrical toner carrying member is covered with the electrode substrate, resulting in poor maintenance performance.